Relaxation Time for Bulk Viscosity of Soft-Sphere and Lennard-Jones Fluids

The density and temperature dependence of bulk relaxation time is studied for soft-sphere and Lennard-Jones fluids. For soft-sphere fluid, the bulk relaxation time is determined by numerical solution of the Zwanzig Mountain equation, using the data of Hansen and Weis for the radial distribution function. The bulk relaxation time for soft sphere fluids decreases monotonically with density and increases with temperature. The bulk relaxation times of Lennard-Jones fluids are determined by numerical integration of the Zwanzig-Mountain equation with the Matteoli-Mansoori radial distribution function. For isotherms up to T^*<2.1, the density dependence of the relaxation time of Lennard-Jones fluids goes through a minimum at a reduced density of approximately 0.85.